1 Abstract: 2 3 Age differences in blood levels of the pituitary hormone thyrotropin (TSH), which 4 regulates thyroid function, may arise from a growing burden of thyroid disease in 5 older adults; but changes may also represent helpful adaptations to aging. It is 6 important for medical practice to know the difference in order to give hormone 7 therapy to those who need it while avoiding harm in those who don?t. Supporting 8 a role for normal aging underlying hormone changes, we know that control of the 9 thyroid shifts in response to alterations in sleep-wake cycle and nutrition that are 10 common with age. At the same time, there is evidence of increased thyroid 11 disease: anti-thyroid antibodies, which can be a marker of damage to the thyroid 12 gland, are more common in older adults. By analyzing simultaneous trends in 13 multiple hormones for 640 participants in the Baltimore Longitudinal Study of 14 Aging with 3-12 years of follow up, we have demonstrated that both age related 15 changes and thyroid disease can be happening in different people with similar 16 TSH levels. Therefore, a single TSH level may not be enough to plan treatment 17 in older adults. We are looking for other tests that could be done to tell the 18 difference between adaptation and disease. We will compare the activity of the 19 TSH hormone, the ability of the pituitary to respond to stimulation, the presence 20 of antibodies, and other blood tests that may serve as markers between these 21 different groups of people. This work is important because when older adults are 22 treated with thyroid hormone un-necessarily, it can do more harm than good by 23 increasing the risk of irregular heart rhythms or bone loss. We know that doctors 24 in the US and the UK have become more aggressive in prescribing thyroid 25 hormone over the past decade, and the number of older adults is growing rapidly, 26 lending increased urgency to this research.